Strength in Numbers: Density-Dependent Volatile-Induced Antimicrobial Activity by Xanthomonas perforans .

For most of the 20th century, was the only known bacterium associated with bacterial spot of tomato in Florida. quickly replaced , mainly because of production of three (BCNs) against ; however, outcompeted even when the three known BCNs were deleted. Surprisingly, we observed antimicrobial activity against in the BCN triple mutant when the triple mutant was grown in Petri plates containing multiple spots but not in Petri plates containing only one spot. We determined that changes in the headspace composition (i.e., volatiles) rather than a diffusible signal in the agar were required for induction of the antimicrobial activity. Other species also produced volatile-induced antimicrobial compounds against and elicited antimicrobial activity by . A wide range of plant pathogenic bacteria, including subsp. , , and , also elicited antimicrobial activity by when multiple spots of the species were present. To identify potential antimicrobial compounds, we performed liquid chromatography with high-resolution mass spectrometry of the agar surrounding the spot in the high cell density Petri plates where the antimicrobial activity was present compared with agar surrounding the spot in Petri plates with one spot where antimicrobial activity was not observed. Among the compounds identified in the zone of inhibition were -butanoyl-L-homoserine lactone and -(3-hydroxy-butanoyl)-homoserine lactone, which are known quorum-sensing metabolites in other bacteria.